1. Field of the Invention
The present invention relates generally to a device and method for diagnosing an evaporative emission control system of an internal combustion engine.
2. Description of the Prior Art
The Environmental Protection Agency (EPA), in a cooperative effort with individual states, automobile manufacturers, manufacturers and contractors of test/diagnostic equipment, develops test procedures and related requirements for use in thoroughly diagnosing the emission systems of motor vehicles. The need for stricter emission system tests, as well as diagnostic instruments to implement such tests, is brought on by the promulgation of newly enacted environmental laws, both federal and state, relating to vehicle emissions. The test procedures and test equipment to be developed to perform the EPA tests reflect a desire by the EPA that sufficient safeguards be in place to prevent false failures, as well as a desire to see enough flexibility in the equipment specifications and quality control requirements to allow for innovative technical approaches to reduce overall costs. These tests are intended to identify a vehicle's true emissions as well as whether the vehicle needs emission repairs. If repairs are needed, the devices used for the tests can be used to ensure that the vehicles are repaired to be in conformance with the requirements.
Since 1971, fuel tanks on cars have been designed as part of a closed system in which vapors that evaporate from the gasoline in the tank are not released into the atmosphere. The system is called an evaporative emission control system and is sealed and under pressure, so that excess vapors are shunted to a charcoal canister known as the evaporative canister. Recent EPA rules require that vehicles pass a purge flow test of the evaporative canister as well as a test that monitors whether pressure in the system is maintained.
The evaporative system purge test is used to determine whether fuel vapor stored in the evaporative canister and present in the fuel tank are being properly drawn into the engine for combustion while the car is being driven. The evaporative emission system uses engine vacuum to draw fuel vapors in the fuel tank, and those temporarily stored in the evaporative canister and attached hoses, into the engine for combustion. The purge flow test determines whether this system is functioning properly by measuring the flow of vapors into a running engine. The pressure test, on the other hand, checks the system for leaks that would allow fuel vapors to escape into the atmosphere.
The purge flow test is generally conducted by driving the vehicle onto a dynamometer, activating vehicle restraints, positioning an exhaust collection device, and positioning an auxiliary engine cooling fan to simulate normal driving conditions. When the purge system is not working properly, the evaporative system can become plugged or perforated or disconnected, resulting in lack of flow to the intake manifold or leaking of hydrocarbons into the atmosphere. In addition to causing hydrocarbon emissions, failure of the purge flow system reduces fuel economy. During the purge flow test, purge flow is measured by simply inserting a flow sensor apparatus at one end of the hose that runs between the evaporative canister and the engine. At present time, EPA rules require that a vehicle have a minimum of 1 liter of flow during a 240-second test in order to pass. Most cars in proper working order will accumulate as much as 25 liters in a minute test cycle, and as much as 100-plus liters over a four-minute transient cycle. As soon as a vehicle exceeds 1 liter of flow, the purge test is complete.
The purge test requires a flow sensor apparatus that can measure the total flow observed over a given transient cycle. Additionally, hoses and universal fittings are required to hook up the flow sensor apparatus. Finally, a metering device is needed to control the test process, collect and record the data, and determine the pass/fail status.
The pressure test monitors for pressure leaks in the system. To check a system for leaks, the vapor lines to the fuel tank, and the fuel tank itself must be filled with nitrogen to a pressure of 14 inches of water (about 0.5 psi), in accordance with present EPA specifications. To pressurize these components, the inspector must locate the evaporative canister, remove the vapor line from the fuel tank near the canister, and hook up the pressure test equipment to the vapor line. After the system is filled, the pressure supply system is closed off, and the drop in pressure is observed. If the system pressure remains above eight inches of water after two minutes, the vehicle passes the test.
A source of nitrogen, a pressure gauge, a valve, and associated hoses and fittings are needed to perform the pressure test. In addition, a metering device is used to automatically meter the nitrogen, monitor the pressure, and collect and process the results. The EPA wants the pressure test performed in less than two minutes on most vehicles. Hence, algorithms must be developed to optimize the test so that a pass/fail decision can be made in less than two minutes.
A number of different devices have been developed to diagnose the purge flow and pressure systems of vehicles. Many of these devices are onboard evaporative emission control devices, permanently coupled to the engine's control module (ECM) to monitor system integrity.
One type of pressure test device is disclosed in U.S. Pat. No. 5,201,212 to Williams, relating to a line leak detector for detecting leaks in underground lines using pressurized nitrogen. A line supplies the pressurized nitrogen, at constant pressure, to the leak tester computation unit and instrument package that supplies the system under test through another line. Selected test system parameters are entered into the instrumentation package prior to running the test. The nitrogen pressure is applied and, during the test, the temperature of the tank and pressure is sampled and the leak rates are compensated for volumetric changes due to temperature.
Similarly, U.S. Pat. No. 5,086,403 to Slocum et al. discloses a microprocessor-based tester that measures the time rate of change of pressure to determine leaks. A leak detector is disclosed therein for a gasoline dispensing system and includes a central monitor and a test probe. The probe has a microprocessor and pressure transducer. The program of the microprocessor considers the pressure versus time signature of the pump system and can compensate for air in the lines and provide gross and precision tests for leaks.
U.S. Pat. No. 5,239,858 to Rogers et al. discloses a method and apparatus for automated testing of a vehicle fuel evaporation control system using an inert gas, such as helium. The system is tested by introducing the inert gas supplied from a cylinder through a pressure regulator and flow sensor apparatus to the fuel filler by use of a cap. The inert gas, helium, introduced to the fuel tank is vented through the system to the vehicle's evaporative canister where it is not absorbed, so it is vented out its perforated bottom and is sensed by a detector, confirming system integrity between the canister and the fuel tank. Starting the engine provides for the inert gas to be drawn from the canister into the engine. The absence of helium at the canister with the engine running would verify operation of the purge system. The helium drawn into the intake manifold would pass through the engine and catalytic converter, and appear in the tailpipe. The mass of helium exiting the tailpipe should equal the mass entering the system through a filler line. Any loss represents leakage in the system.
Conventional pressure testers, such as those described above, depend on constant flow pressurization techniques which are fundamentally inadequate for two reasons. First, a pressurizing scheme based on constant flow pressurization takes a prohibitively long time to perform. Second, the use of a constant flow pressurization technique does not lend itself to practical application in pressure testing of an evaporative emission control system. The high level pressurization requirements, as defined by EPA specifications, demand nitrogen pressurization to levels as high as 0.5 psi in a very short time frame.
Likewise, conventional purge testers operate to detect the proper functioning of a vehicle purge system as accomplished at various time intervals and at known rates of flow. To date, none of the modern-day purge testers are capable of measuring rates of fuel flow in the range of zero to 60 liters/minute as required by the EPA.
The EPA now requires that purge flow be detected for values as low as 1 liter over a four minute time cycle. This is equivalent to an average flow rate of 0.25 liters/minute. Conventional purge flow testers are incapable of measuring fuel vapor flow rate at such low levels.
Furthermore, there has never been a single, self-contained, portable, evaporative emission tester capable of performing both purge flow and pressure testing.